In TypeScript, why is the function return type not assigned to generic type as expected?-CodePudding

I have this code in TypeScript and the ParsedData generic type is not populated correctly. I don't understand what's preventing TS from inferring the correct type below.

function fromData<RawData, ParsedData>(rawData: RawData) {
    return function withParser(
        parser: (data: RawData) => ParsedData,
        renderer: (parsedData: ParsedData) => unknown
    ) {
        const parsedData = parser(rawData)
        
        return {
            renderedData: renderer(parsedData),
            parsedData
        }
    }
}

const hasData = fromData({ user: 1 })

hasData(
    (rawData) => rawData.user   1,
    (parsedData) => parsedData   100 // PROBLEM: 'parsedData' is of type 'unknown'.(18046) 
)

In general, I would like to call a function with some data and receive another function that can be called at a later time with a parser and renderer arguments that infer correct types throughout.

I'm expecting to see parsedData to be correctly typed in my IDE, so that I can more easily implement a renderer function using the return type of the parser function.

CodePudding user response：

This happens because ParsedData is a type parameter of fromData, and at the fromData call the ParsedData type has not been determined yet. You can fix it by instead parameterizing the returned function with ParsedData:

function fromData<RawData>(rawData: RawData) {
    return function withParser<ParsedData>(
        parser: (data: RawData) => ParsedData,
        renderer: (parsedData: ParsedData) => unknown
    ) {
        const parsedData = parser(rawData)

        return {
            renderedData: renderer(parsedData),
            parsedData
        }
    }
}

const hasData = fromData({ user: 1 })

hasData(
    (rawData) => rawData.user   1,
    (parsedData) => parsedData   100
    // (parameter) parsedData: number
)

TypeScript playground